We are in the process of getting quotes to have our furnace and air conditioner replaced. One of the guys mentioned that the airflow in the house is not efficient because our air return duct is about 4 inches smaller than the outgoing duct. Is this something that makes sense?
WAG alert!
The return needs to supply the air for the vents, and logically can’t put out more than it can take in. Thinking about it further, could the return simply be a more powerful intake than the vents are outputting?
I’ve heard it said that you can’t give (ie impossible) the furnace too much air. One solution (probably his) is to replace them with matching air ducts. Another is to put a fan in the ducts (you can get them that are incorporated to a piece of ductwork).
[disclaimer]
the only reason I know anything at all like this is that I have to upgrade ours…the basement cold air returns are in the ceiling! I have to drop a couple of ducts to the floor to draw off the cold air rather than sucking up the warm air I just pumped in!
[\disclaimer]
My guess is that the return ducts may be a bit too small but I doubt it makes sense to change them now. New design and construction is one thing but if it is just the size (not placement or other factors) the fan wil just work a bit harder or you may want a more powerful fan but unless the design is awful it would not make sense to change the ducts now. Of course, the installer has an interest in convincing you other wise. Absent more details I would caution you to get a second opinion.
You know more about your house than anyone else. Was the heating adequate before? Are you intending to put in the same size furnace and AC as before? If so, and things worked before why shouldn’t they work now? If you are installing bigger units with bigger blowers, all of your ducting will be smaller than optimum and you probably won’t get as much air flow as you expected. That might not be bad, but it is what would happen.
Some place like Home Depot probably has some info on duct sizing.
The intake should be bigger then the exhaust but you may be able to make a small modification to it (add an additional intake) to compensate.
Supply ducts are typically smaller because they are sealed and can run at higher pressures and air velocities. Returns are larger because you want to minimize the friction losses to keep the fan supplied with air.
…so yes it makes engineering sense. Whether it makes economic sense is another matter entirely.